1. Field of the Description
The present invention relates to a liquid crystal display device and a method for manufacturing the same. More particularly, the present invention relates to a liquid crystal display device including a display medium sandwiched between a plasma generation substrate having channels for generating plasma and a counter substrate, and a method for manufacturing the same.
2. Description of the Related Art
There is a demand for increasing a viewing angle of a liquid crystal display device because the viewing angle of a conventional liquid crystal display device is rather narrow. As a method of increasing the viewing angle of a twisted nematic liquid crystal display device (TN-LCD), the following techniques are conventionally adopted: (i) A technique of attaining a wide viewing angle by improving the alignment state of liquid crystal in a liquid crystal cell (namely, a technique for making improvement from inside the liquid crystal cell); and (ii) a technique of attaining a wide viewing angle by spreading beams on the side of a viewer by utilizing the characteristic of the front face of a liquid crystal cell (namely, a technique for making improvement from outside the liquid crystal cell).
In adopting the technique of attaining a wide viewing angle by improving the alignment state of the liquid crystal in a liquid crystal cell, the viewing angle characteristic can be improved by aligning liquid crystal molecules in at least two directions within a pixel. This technique will now be described referring to FIGS. 12A through 12F.
FIGS. 12D through 12F are schematic sectional views for showing the alignment state of the liquid crystal molecules in a conventional TN-LCD, wherein FIG. 12D shows the alignment state under application of no voltage, FIG. 12E shows the alignment state in a gray level display under application of a voltage, and FIG. 12F shows the alignment state under application of a saturation voltage. This liquid crystal display device has a pretilt angle in its initial alignment under application of no voltage as shown in FIG. 12D. When a voltage is applied, the liquid crystal molecules all rise in the same direction as shown in FIG. 12E. Accordingly, when the liquid crystal display device is viewed at different angles A and B, the display contrast varies greatly because the apparent refractive index is different between these angles. In addition, in the gray level display as is shown in FIG. 12E, the display quality is largely degraded; for example, a displayed image can be inverted depending upon the viewing angle. FIGS. 12A through 12C are schematic sectional views for showing the alignment state of liquid crystal molecules in a liquid crystal display device in which the liquid crystal molecules are aligned in at least two directions; wherein FIG. 12A shows the alignment state under application of no voltage, FIG. 12B shows the alignment state in gray level display under application of a voltage, and FIG. 12C shows the alignment state under application of a saturation voltage. When this liquid crystal display device is viewed at the angles A and B, the apparent refractive index is averaged so that the light transmittance can be the same at both angles, and hence, the viewing angle characteristic can be improved in the gray level display as well.
As specific examples of a liquid crystal display device with a wide viewing angle, the following six types are known:
A first type liquid crystal display device includes a polymer wall in a liquid crystal cell, and requires neither a polarizing plate nor an alignment treatment. In this liquid crystal display device, the transparent and opaque states are electrically controlled by using the birefringence of liquid crystal. In this type, the refractive index against ordinary light of the liquid crystal molecules accords with the refractive index of a support medium. Therefore, the transparent state is basically displayed when a voltage is applied to align the liquid crystal, and the opaque state (that is, the light scattering state due to the turbulence of the alignment of the liquid crystal molecules), is displayed when a voltage is not applied. Various techniques have been proposed regarding this type of liquid crystal display device. For example, Japanese laid-open National Patent Publication 61-502128 discloses a technique of forming liquid crystal droplets in a photopolymerizable resin or a thermosetting resin by polymerizing the resin in a mixture of the resin and liquid crystal and depositing the liquid crystal. Furthermore, Japanese Laid-Open Patent Publication Nos. 4-338923 and 4-212928 describe a technique of attaining a wide viewing angle by providing two orthogonal polarizing plates on both outer surfaces of a liquid crystal cell.
In a second type liquid crystal display device, in order to improve the viewing angle characteristic of a liquid crystal cell of a non-scattering type using a polarizing plate, phase separation is caused in a mixture of liquid crystal and a photopolymerizable resin, thereby forming a composite material of the liquid crystal and the polymeric material (Japanese Laid-Open Patent Publication No. 5-27242). In this type liquid crystal display device, the composite material makes the alignment of liquid crystal domains random, and hence, the liquid crystal molecules rise in different directions in the respective domains when a voltage is applied. As a result, the apparent transmittance becomes the same at various viewing angles, namely, the value of d.multidot..DELTA.n (where-in d indicates the thickness of a liquid crystal layer and .DELTA.n indicates the refractive index anisotropy of the liquid crystal) is averaged, and hence, the viewing angle characteristic in the gray level display can be improved.
A third type liquid crystal display device has recently been proposed by the present inventors, in which liquid crystal molecules are omnidirectionally (for example, spirally) aligned in a pixel region. This liquid crystal display device is manufactured by light irradiation of a liquid crystal cell including a liquid crystal material and a photopolymerizable resin two-dimensionally controlling a light intensity distribution within a screen (for example, by using a photo-mask). The liquid crystal molecules are controlled in accordance with a voltage to be applied, so that the spiral alignment becomes similar to the homeotropic alignment, and hence, the viewing angle characteristic can be remarkably improved (Japanese Laid-Open Patent Publication Nos. 6-301015 and 7-120728).
In a fourth type liquid crystal display device, a film of a crystalline polymeric material having a spherulite structure is formed on a substrate. A wide viewing angle display mode can be attained by utilizing an axisymmetrical alignment regulation force caused by the spherulite structure (Japanese Laid-Open Patent Publication No. 6-308496).
In a fifth type liquid crystal display device, an alignment film is coated on a substrate, and liquid crystal molecules are aligned in random directions without conducting an alignment treatment such as a rubbing treatment (Japanese Laid-Open Patent Publication No. 6-194655).
In a sixth type liquid crystal display device, a pixel region is divided into a plurality of areas, and liquid crystal molecules are aligned in each area so as to compensate the viewing angle characteristic (Japanese Laid-Open Patent Publication No. 57-186735).
Meanwhile, as a liquid crystal display device (LCD) for a plane display, a TFT-LCD including a thin film transistor (TFT) as a switching element for turning on/off pixels is widely used at present.
The TFT-LCD is, however, regarded as not practical for use as a display apparatus with a large screen of 20 inches or more, for example, a wall-mounted television, which is expected to be realized in the future. In the field of the display apparatus with a large screen, a plasma address LCD (PALC) is proposed as one of replacements of the TFT-LCD (Japanese Laid-Open Patent Publication No. 1-217396).
The plasma address LCD comprises, as is shown in FIG. 13, a transparent substrate 101 and a plasma generation substrate 111 for sandwiching a liquid crystal layer 102. The plasma generation substrate 111 includes walls 112, a thin glass plate 116, line-like channels 113 defined by the walls 112 and the thin glass plate 116 for sealing an ionization gas therein, and anode electrodes 114 and cathode electrodes 115 disposed in the channels 113 for ionizing the ionization gas.
On the surface of the substrate 101 facing the liquid crystal layer 102, transparent electrodes 105 serving as data lines are formed in the shape of stripes extending, when seen in the normal direction of the substrate, in the perpendicular direction to the channels 113.
This plasma address LCD is driven by successively scanning the channels 113 and supplying image data to the corresponding transparent electrodes 105 synchronously with the scanning of the channels 113.
For example, as is shown in FIG. 14A, the channels 113 are successively scanned, so that a selected channel 113 is ionized to be turned on. Under this condition, the voltage variation of data 1, 2 or 3 supplied via the transparent electrodes 105 is held by storing charges in the rear surface (i.e., the surface not facing the liquid crystal layer) of the thin glass plate 116. On a channel 113 that has been ionized (i.e., in an on-state), a signal from the corresponding transparent electrode 105 is applied to the liquid crystal layer 102, so that the transmittance of the liquid crystal layer 102 is changed. On a channel 113 that has not been ionized (i.e., in an off-state), the transmittance of the liquid crystal layer 102 is not changed. In this manner, a switching operation for turning on/off the liquid crystal cell is conducted by ionizing or nonionizing each channel 113 as is shown in FIG. 14B. Thus, an image can be displayed.
In order to manufacture a display apparatus with a large screen adopting the aforementioned display technique at a low cost, it is necessary to manufacture the channels 113 by an economical method. As a method for manufacturing the channels at a low cost, a technique of forming walls or the like by printing glass paste on a glass substrate is proposed (Japanese Laid-Open Patent Publication No. 4-265931).
However, the above-described plasma address LCD (PALC) has the following problems:
First, the liquid crystal display mode used in the PALC is mainly the TN mode. When a display device with a large screen is manufactured by using the TN mode, the viewing angle can be different, for example, like angles a and b shown in FIG. 15, depending upon the position of a viewer. In other words, there are a number of positions of a viewer having largely different viewing angles on the screen. Therefore, an image can be differently seen from the positions having the different viewing angles when the TN mode is adopted. Furthermore, in using such a large screen, the thickness of the liquid crystal cell is varied due to the weight of the liquid crystal in the liquid crystal layer when the cell is manufactured by the conventional method in which beads are disposed in the liquid crystal layer for retaining the thickness at a predetermined size. In particular, when a plastic substrate is used or a polarizing plate is used also as a substrate for the purpose of decreasing the weight and the thickness of the display device, the cell thickness can be largely varied.
Second, in a liquid crystal cell of the TN mode, a polarizing plate is occasionally disposed with its polarizing axis inclined at an angle of 45.degree. against the latitudinal or longitudinal direction of the screen because of its viewing angle characteristic. In this case, there arises a problem that a point for adhesion can be seen so as to cause light leakage because of birefringence at a point having a different refractive index (for example, an adhesion point between a plasma generation substrate and a thin dielectric film) or because of a difference in the reflectance of polarized light at the adhesion point.
Thirdly, when a dielectric film (for example, a glass plate) disposed between a plasma generation substrate and a liquid crystal layer is made thick in order to improve the strength of such a display device, it is difficult to control the position of charges on the side of the film facing the liquid crystal layer, and hence, a resultant displayed image is unclear with a blurred outline. In order to solve this problem, a technique of forming transparent electrodes on the surface of the glass plate facing the liquid crystal layer in a pattern in accordance with the direction of the channels is proposed (Japanese Laid-Open Patent Publication No. 4-313788). This technique has, however, the following problems. Since this technique includes the step of forming an ITO film on a thin glass film and patterning it, the manufacturing PROCESS would be complicated. Furthermore, the need of alignment with a black mask complicates a fabrication apparatus for a liquid crystal display device. In contrast, when the dielectric film is made thin, the strength of the display apparatus is insufficient, and the cell thickness in the liquid crystal layer is varied so as to cause nonuniformity in a displayed image. In an operation mode with an alignment film, particularly in the case of a liquid crystal panel with a large-sized screen, a voltage applied across a liquid crystal layer (a voltage distribution) varies from part to part due to nonuniformity of the thickness of the alignment film, resulting in a nonuniform displayed image.
Fourthly, a rubbing treatment is required in the TN and STN modes where liquid crystal molecules are aligned in one direction on the substrate, and the rubbing treatment can damage the complicated structure of a plasma generation substrate. Furthermore, when beads are used for retaining the cell thickness, a stress is collected on portions in contact with the beads during the fabrication of the cell and the injection of the material for a display medium by vacuum injection. This stress can occasionally damage the glass plate disposed between the channels and the display medium.
Accordingly, it is desired to realize a liquid crystal display device applicable to a large screen, having a fixed cell thickness, a high strength (for example, whose member disposed between plasma generation channels and a display medium can be prevented from being damaged) and a wide viewing angle characteristic, as well as a simple method for manufacturing such a liquid crystal display device.